We tested https://wordpress.org/plugins/formidable/ on our test site and found no issues with the site display. We used formidable form on the sidebar text widget. Category archives widget was also working fine.
We want information on how to replicate your issue on our test site which we require for troubleshooting and fixing the conflict if any.
Also:
– Disable all other plugins to check conflict with among other plugins.
– Disable caching plugin during troubleshooting if installed.
Please send email to [email protected] with screenshots of the issue and details on how to replicate your setup on our end.